GASTROPODS . 
3<->9 
Mediterranean, and it probably also occurs in the Atlantic. The second species, of 
which only the shell is known, was found in New Zealand. Cymbuliopsis and 
Gleba are the two remaining; 
© 
genera of this family. 
Front - Gilled Group,— 
Order Prosobranchiata. 
The Prosobranchs, which 
include the majority of 
marine gastropods and a 
few groups of terrestrial 
forms, are always contained 
within or protected by a 
shell, and organised for 
crawling. The branchial 
or pulmonary cavity is in 
advance of the heart, and 
the auricle of the latter in front of the ventricle. This important anatomical 
character, which is expressed or referred to in the term Prosobranchiata, can best 
be understood by observing the accompany¬ 
ing figure representing the male of one of 
the periwinkles ( Littorina ). Here the 
respiratory organs are lodged in a chamber 
formed by the mantle behind the head ; the 
gills are variable in the different suborders. 
In some forms, with nonspiral shells, they 
are double and symmetrical, but in others, 
and these are the majority, where the shell 
assumes a spiral form, the gill is usually 
single. Some of the Prosobranchs have 
horny jaws, but others are without; most 
are furnished with a radula, but this is 
sometimes absent. Characters derived from 
this organ have been largely employed in 
the classifications of the suborders. The 
sexes are separate; the head is distinct, and 
furnished with a pair of tentacles, sometimes 
having the eyes at the ends, but mostly at or 
near their base. An operculum, which is 
unknown among the Opisthobranchs and 
a, Moutli; b, Sexual organ ; c, Reflexed mantle ; PullllOliateS, except ill the Case of Amphibolci, 
h, Heart; m, Shell-muscle; p, Foot. is developed in the majority ot Prosobranchs, 
but the number of genera in which this 
appendage is absent is considerable. It assumes very different forms, and its size is 
very variable in proportion to the dimension of the aperture of the shell. In some 
vol. vi.—24 
ANIMAL OP MALE PERIWINKLE, WITH THE SHELL 
REMOVED. 
Gleba cordata. a. Body ; b, Fin ; e, Proboscis. 
